The present disclosure relates generally to methods of real time display of filtered waveform data and, more particularly, to a method and apparatus for real time display of filtered electrocardiogram data.
An electrocardiogram (ECG) of a cardiac cycle is detected across sense electrode pairs located on the surface of a patient's body, and is a repetitive waveform characterized by a periodic PQRST electrical activation sequence of the upper and lower heart chambers. The PQRST sequence is associated with the sequential depolarization and contraction of the atria followed by the depolarization and contraction of the ventricles, and successive PQRST complexes are separated by a baseline or isoelectric region.
As shown in FIG. 1, The PQRST electrical activation sequence commences with the P-wave, which is indicative of the depolarization and contraction of the atria. Following is the QRS complex, which is indicative of the depolarization and contraction of the ventricles. The T-wave at the termination of the ST segment time delay is associated with re-polarization of the ventricles. The PQRST electrical activation sequence with intact A-V activation detected across a sense electrode pair is fairly predictable in shape. The P-wave, R-wave and T-wave events occurring in sequence in the range of normal heart rates are usually readily recognized by visual examination of the external ECG recorded by applied body surface electrodes that are correctly oriented with the depolarization waves. The P-wave and the R-wave are readily sensed by sense amplifiers of a monitor or therapy delivery device coupled with appropriately placed sense electrode pairs.
The ST segment of the ECG is typically close in amplitude to the baseline or isoelectric amplitude of the signal sensed between PQRST sequences, depending on the sense electrode pair location. During episodes of myocardial ischemia, the ST segment amplitude is elevated or depressed (depending on positioning of the ECG sense electrodes in relation to the heart) from baseline. These ST segment deviations can be readily recognized by visual examination.
However, the ECG signals are typically subject to low frequency noise (such as, for example, from respiration that occurs at a lower rate than the heart rate), thus resulting in baseline drift. Such an effect can render the ECG waveform difficult to read, especially in a display device having multiple ECG waveforms presented simultaneously. Presently, there are filtering techniques in existence that aggressively remove the baseline drift, but which also result in a distorted portion of ECG waveform (e.g., the ST segment) and/or introduce a delay in the display presentation. If a filtering technique is aimed at minimizing the distortion or eliminating a delay, this typically comes at the price of not aggressively correcting the baseline. Accordingly, it is desirable to be able to compensate for baseline drift and low frequency noise, while maintaining the integrity of the ECG waves and complexes without introducing a delay in the display thereof.